Computer Networks ICS 651. IP Routing RIP OSPF BGP MPLS Internet Control Message Protocol IP Path MTU Discovery

Transcription

1 Computer Networks ICS 651 IP Routing RIP OSPF BGP MPLS Internet Control Message Protocol IP Path MTU Discovery

2 Routing Information Protocol DV modified with split horizon and poisoned reverse distance vector algorithm, using hop count as metric Intradomain routing only (e.g. within hawaii.edu) RIP v2 allows specification of netmask RIPng (RFC 2080) supports IPv6 routers exchange messages every 30 seconds, and when their routing tables change

3 Open Shortest Path First OSPF LS using reliable flooding based on sequence numbers used for routing within an autonomous system or domain, which may be subdivided into areas supports load balancing and multiple metrics OSPF v3 (RFC 5340) supports IPv6

4 IP Routing: BGP-4 Border Gateway Protocol Internet is an arbitrarily connected set of autonomous systems a Stub AS has single connection to Internet a Multihomed AS has multiple connections but refuses to carry transit traffic a Transit AS will carry transit traffic try to find a route, don't try too hard to find the optimal route one router in AS is the BGP speaker, advertises reachable networks not DV or LS: instead, protocol distributes paths to destinations a path vector algorithm explicit cancellation: withdrawn routes

5 BGP Policy RIP is very easy to configure, because there are no options OSPF usually works on networks subdivided into areas, so some configuration is necessary BGP allows the network administrator to specify which transit AS is preferred over others, so can require a lot of configuration policy preferences allow selecting or preferring paths that do not cross expensive links and have the least security concerns Autonomous Systems truly are autonomous, so each administrator selects the policies that are best for this AS

6 MPLS MPLS adds to each packet a small header which includes an integer label each router in a transit AS can be configured to recognize this label and forward the packet, unmodified, to a specific interface and next hop this Label Switching is very simple and can be implemented in hardware label switching must be set up by software that computes a path through the network for each pair of source (ingress) and destination (egress) routers, assigns a different label to each path, and distributes to each router the labels and the relevant details of each path MPLS can support any protocol, not just IP

7 Other Routing Protocols IS-IS is another link-state protocol, typically working at the layer below IP, and so, like MPLS, more protocol-agnostic than OSPF or RIP EIGRP is a Distance-Vector protocol, but sends updates instead of the entire routing table. EIGRP was proprietary to Cisco until 2013, when (most of) the protocol was published as RFC 7868

8 Routing Protocols Summary Interior Gateway Protocols, or IGPs, route packets within an autonomous system Exterior Gateway Protocols, or EGPs, route packets between autonomous systems there is only one common EGP, which is BGP. All the other routing protocols we have discussed are IGPs in general, each autonomous system will be running just one IGP, and may have one or more BGP routers that are used as gateways to other autonomous systems

9 IP Routing Summary summarize, summarize, summarize use hierarchy whenever possible: route to networks, not hosts OSPF: route to areas, not networks BGP: route to autonomous systems routing protocols can evolve faster than the underlying IP transport Interior Gateway Protocols (IGP) are the most automated: RIP for small networks, OSPF and IS-IS for larger ones.

10 Internet Control Message Protocol ICMP the Internet is complex how do we find out what is going wrong? send a packet "there and back": ICMP echo, ping send an ICMP error packet whenever we drop a (non ICMP error) packet ICMP: RFC 792

11 ICMP Echo Echo packet (type 8) or Echo Reply (type 0) checksum covers entire packet identifier (typically process ID on sender machine) sequence number (typically 1, 2, 3...) arbitrary data follows (could be large) for ping, data typically holds binary date and time (2 32-bit words, 8 bytes)

12 Other ICMP Types [3] Destination Unreachable (network, host, protocol, prohibited...) [11] Time Exceeded (in transit, during reassembly) [5] Redirect: use this other router for this destination [9] Router advertisement [10] Router solicitation [4] Source Quench [12] Parameter Problem

13 IP Path MTU discovery send IPv6 packets, or IPv4 packets with DF (Don t Fragment) set cannot send more than local network MTU if the router must drop a packet that exceeds the MTU of the outgoing interface, it can send a "destination unreachable/fragmentation needed" ICMP message, or an ICMPv6 packet too big message this ICMP message carries the MTU if there is no ICMP message, sender can do a binary search (on common MTU sizes) to find an MTU that works however, the path MTU can change! since ICMP message may be dropped, also needs other ways to detect dropped packets slow, time-consuming, error-prone...